.beta.-Isophorone is a synthetic intermediate in the manufacture of carotenoids, of vitamins, such as vitamin E, and of pharmaceuticals.
It is also directly involved in syntheses of fragrances and natural products, such as astaxanthin and abscisic acid and derivatives.
.beta.-Isophorone is an isomer of .alpha.-isophorone, obtained by trimerization of acetone in alkaline medium, which is distinguished from the latter by the position of the double bond: the double bond is no longer conjugated with the carbonyl, as represented hereinbelow: ##STR1##
The isomerization of .alpha.-isophorone to .beta.-isophorone is an equilibrium deconjugation reaction of the double bond and of the carbonyl and, for this reason, the thermodynamic equilibrium is weighted towards .alpha.-isophorone.
Numerous processes for the isomerization of .alpha.-isophorone to .beta.-isophorone have been described but exhibit numerous disadvantages, such as high consumption of chemicals (in particular catalysts), mediocre yields or formation of .alpha.-isophorone condensation products (heavy products), which results in a rise in the temperature of the reaction mixture,. thus accelerating the formation of heavy products, which destabilize the system.
Patent EP 832,871, Example 7, discloses a process for the preparation of .beta.-isophorone by catalytic isomerization of .alpha.-isophorone which consists, in a first step, in continuously extracting from the reaction mixture, by distillation, a primary mixture comprising from 20 to 22% of .beta.-isophorone and in then isolating from this mixture, by distillation, a .beta.-isophorone with a purity equal to approximately 98%. It is found, when proceeding in this way, that a mixture composed of approximately 90% of .alpha.-isophorone and 10% of heavy products is collected in the recirculation evaporator. In view of the duration of operation (approximately 15 hours) and of the amount of product collected in the said evaporator (615 g), this corresponds to an hourly production of heavy products of approximately 4 g/h.
In addition, a large amount by weight of catalyst is used: 0.6% with respect to the .alpha.-isophorone employed.
If the thermodynamics of the reaction are considered, it is found that the temperature at which isomerization is carried out fixes the concentration of .beta.-isophorone at equilibrium: the higher the temperature, the greater the concentration at equilibrium of .beta.-isophorone. As .beta.-isophorone is more volatile than .alpha.-isophorone, the equilibrium can be shifted by distillation of the most volatile product. In addition, .beta.-isophorone, on heating, reverse isomerizes to .alpha.-isophorone, even in the absence of catalyst: the kinetics of this thermal reverse isomerization are slow.
It is therefore advantageous to carry out the isomerization at high temperature.
The objective of obtaining pure .beta.-isophorone (.beta.-isophorone/.alpha.-isophorone.gtoreq.2 99%) cannot be achieved in a single column at the isomerization temperature. This is because the .beta.-isophorone/.alpha.-isophorone separation requires not only a column with many theoretical plates but also a high reflux ratio. The reflux ratio R is defined as being the ratio of the total amount of condensate obtained at the distillation column top to the amount of product extracted. The increase in the reflux ratio makes it possible to increase the concentration of the .beta.-isophorone extracted up to a certain threshold. This is because a high reflux ratio increases the mean residence time of the .beta.-isophorone in the distillation column, that is to say at high temperature, promoting thermal retrogression.
Thus, a high reflux ratio does not make it possible to produce .beta.-isophorone referred to as pure.
Furthermore, to carry out the isomerization, it is necessary to pass through the intermediate enol or enolate, which are also intermediates in the aldolization/crotonization reaction, which results in the formation of isophorone polycondensates which constitute heavy products, the formation of which is to be prohibited since they represent a loss of isophorone in a process for the manufacture of .beta.-isophorone. The formation of the heavy products is promoted, on the one hand, by the concentration of catalyst and, on the other hand, by the temperature.
Thus, the use of heterogeneous catalysts does not appear to be very favorable as it can increase the concentration of heavy products at the surface of the solid catalyst, which can deactivate the said catalyst.